Method and apparatus for sterilizing by electron bombardment



March 1957 J. G. TRUMP 2,785,313

METHOD AND APPARATUS FOR STERILIZING BY ELECTRON BOMBARDMENT Filed July26, 1952 Unite rates lVIETl-IOD AND APPABLATUS FQR STERELEZWG BYELECTRQN EQMBARBMENT Application July 26, 1952, Serial No. 361,111

8 illairns. (Cl. 259-495) This invention relates to electronsterilization, and particularly to a simple method of and severalsubstantially different apparatus for increasing the amount of ionizingenergy absorbed by an object to be sterilized from an available sourceof high-energy electrons. Still more specifically and particularlystated, my invention relates to the more uniform administration ofionizing energy to object irradiated by high-energy electrons which Ipreferably accomplish by substantially surrounding or enclosing saidobject with a metal of high atomic number, so as to redirect towardssaid object both those electrons which have passed alongside it and alsothose electrons which have been scattered from its volume.

it is now becoming established that all types of living organisms areaffected by gamma rays and high energy electrons, and that lethaleffects can be produced on unwanted organisms by doses which will raisethe temperature of water only a few degrees centigrade. The growingavailability of streams of high energy electrons makes possible thepractical application of this knowledge to the sterilization of manyimportant products, such as pharmaceuticals, surgical instruments,animal tissue for transplant purposes, as well as for the preservationof certain foods. Only high energy electron sources, as distinct fromgamma ray sources, appear to possess enough total power output to handleeconomically the considerable amounts of material which may requiresterilization.

Measurements of the properties of high energy electrons have disclosedthat their range in typical materials is small compared to of gammarays. A Z-millionvolt electron has a maximum range in water of 1 cm.Beyond this limiting distance there is no biological effect, while themaximum biological effect occurs at one-third this range. Althoughpractical high energy electron sources may be constructed for manymillions of volts, such higher energy apparatus becomes progressivelymore expensive and also often has a lower output electron currentcapacity.

A common method of, in efiect, doubling the range of penetration of anavailable stream of electrons is to irradiate the object from bothsides. This can be done in principle by reversing the object andirradiating again, or by irradiating the object simultaneously from twoelectron sources.

In many cases such methods of securing the doubling of the electrontreatment range are uneconomic and excessively complex. One obiect of myinvention herein disclosed and claimed is to double the effective rangeof penetration of an available stream of electrons in a simple manner. Ihave discovered that if a primary stream of electrons is directed fromone aspect or source or direction against an object to be sterilized,but some of said electrons are nevertheless allowed to pass said objectand to strike a high atomic number material positioned immediatelybeyond or adjacent to said object, then a proportion of electrons willbe electrostatically scattered backward by said high atomicnumbermaterial and serve are 1:

the purpose of simultaneous irradiation of said object rom the oppositeaspect.

Although it is known that the effective range of penetration of anavailable strearn of electrons may be doubled by irradiating an objectfrom opposite sides, my he ein disclosed invention has the advantage ofsimplicity over other proposed methods. My invention has the furtheradvan age of simultaneity. It is often not practical to irradiate anobject from one aspect and then a few moments later to complete theirradiation from a second aspect. This is particularly true in the caseof liquids.

Another object of my invention herein disclosed is to irradiate complexshapes with a stream of electrons in a simple manner, since theelectrons which pass the object to be sterilized and are reflectedbackward by the high atomic number material are scattered at manyangles. My invention would, for example, be particularly useful in theirradiation of a tray of surgical instruments where complete electronsterilization of even apparently inaccessible surfaces is required.

Still another object of my invention is to render the dose distributionmore uniform near the surface of an object to be sterilized by highenergy electrons. Most materials to be sterilized are of relativelylow/atomic number, but even so, five or ten percent of the incidentenergy may be reflected backward from its upper surface. By placing ahigh-atomic number material above an object to be sterilized andalongside of the incident beam of high energy electrons, l am able toreflect back toward the object some of those electrons which have beenscattered out of it. The reflected electrons have the efiect of addingdosage to the low dosed region which always exists near the surface ofthe object to be sterilized, so that by my invention 1 may reduce oravoid the necessity of other methods of rendering the dose distributionmore uniform near the surface.

These and other objects of the invention, both as to method and toapparatus, will be best understood by reference to the followingdescription when taken in connection with the accompanying drawing,while its scope will be more particularly pointed out in the appendeclaims.

in the drawing:

Fig. 1 is a diagrammatic view in vertical cross-section of one form ofapparatus for doubling the effective range of an available stream ofelectrons;

Fig. 2 is a similar view illustrating one'forrn of ap paratus forrendering the dose distribution more uniform near the surface of anobject to be sterilized by high'energy electrons;

Fig. 2A is a side elevation of the apparatus shown in Fig. 2;

Fig. 3 is a similar view illustrating a modification of the apparatusillustrated inFigs. l and 2;

Fig. 3A is a side elevation of the apparatus shown in Fi 3; and

Fig. 4 shows in vertical section a portion of an acceleration tube forthe passage of a beam of high energy electrons.

Referring to the drawing, and first to Fig. 1, the arrow 1 represents astream of high energy electrons; My invention may be practiced toadvantage with any source of high energy electrons. However, in theembodiments of my invention which are illustrated in Figs. 2 and 3, 1preferably employ a well-donned beam of electrons such as 1, in orderthat the reflector 4a in Fig. 2 or 412 in Fig. 3 (to be described indetail hereinafter) may not interfere with the stream of high energyelectrons .1, Such a well defined beam of electrons, may be created byand discharged, from apparatus: such as disclosed in patents issued inmy name as inventor or (to-inventor,

for-example, and/or apparatus manufactured by High Voltage EngineeringCorp. of Cambridge, Massachusetts, whereby the electron beam may beaccelerated and strike the object with the full energy, as shown, forexample, in Fig. 4 at A. Referring again to Fig. 1, an object 2 to besterilized is placed upon any suitable support 3, so that said object 2is in the path of the stream of electrons 1 issuing as a well-definedbeam from a suitable source. Immediately below the support 3 there isshown in Fig. 1 a reflector 4 which may be a plate of any material, butwhich is preferably a metal of high atomic number.

I have discovered from experiments made by me that a fairly highproportion of primary electrons bombarding a material of high atomicnumber are reflected with most or all of their initial energy. Many ofthese'high energy but now secondary electrons are reflected directlybackward, but statistically they are reflected over a wide band ofangles, depending primarily upon the atomic number of the reflectingmaterial and the energy of the primary electrons. It can thus be seenthat if, as shown in Fig. 1, the stream of electrons 1 is directedagainst the object 2, but some of said electrons are allowed to pass theobject 2 and to strike the reflector 4, then a proportion of saidelectrons will be electrostatically scattered backward, as indicated bythe arrow 5, and serve the purpose of simultaneous irradiation of theobject 2 from the opposite aspect or direction.

Suitable high-atomic-number materials for the reflector 4 are tungsten,gold, lead, and uranium. Lower atomic number metals may be used, but theproportion of reflected high energy electrons will be less, asdetermined from my experiments.

, In general, the reflector 4 is stationary, although it may be inmotion, as shown at 4c in Fig. 3 yet to be described. If the reflector 4is stationary, the constant bombardment thereof by high energy electronswith the consequent absorption of large amounts of energy may result inexcessive heating of such reflector 4. In order to carry away the energyabsorbed by the reflector 4 in this process, I may water or air cool asby a flowing system, the reflector 4 by any appropriate apparatusindicated at 4.

The mechanism or property 'or force of matter whereby a high energyelectron may be reflected with little or no loss of energy from a targetmaterial is known as elastic nuclear scattering. This type of scatteringresults from an interaction between an arriving high energy electron andthe field of force of the nucleus of the target atoms. comet arriving inthe solar system and the gravitational .field of the .sun. Both thearriving electron and the arriving comet experience a force ofattraction which increases inversely with the square of the distancefrom the nucleus of the target atoms and the sun, respectively. Both theelectron and the comet travel around the nucleus and the sun,respectively, so that they are in effect reflected back towards thedirection from which they came. The electron-nucleus interaction is themore vigorous the higher the atomic number. of the target atoms, and itis primarily for the highest atomic number metals that a sufficientreflection of primary energy in the opposite direction can be obtainedto justify economically the herein disclosed and claimed procedure. Inprinciple, however, even lower atomic numbered materials, such asaluminum, may eflectively be used.

The back-reflected electrons are scattered at ,many angles, so that myinvention herein disclosed is particularly advantageous in theirradiation of objects of complex shape as, for example, a tray ofsurgical instruments.

In another embodiment of the apparatus of my invention, and by which mymethod may be practiced, I may use metals of high atomic number toreflect back toward an object to be sterilized, those electrons whichhave been scattered out of it. 1 1

This interaction is similar to that between a Referring now to Fig. 2,therein is shown an object 2 to be sterilized, which is substantiallysurrounded and wholly enclosed by a high atomic number metal reflector4a, except for an aperture 7 through which the stream of electrons 1 isdirected from a suitable source. The stream of electrons lis preferablywell-defined in order to avoid bombardment of the upper exterior surfaceof the reflector 4a. The object 2 may rest upon any convenient support3, and as indicated, the upper part of the enclosing reflector 4a isformed as a removable cover 4x to permit the object to be positioned.Any electrons which are scattered out of the object 2 are reflected backtowards said object 2 by the reflector 4a, as indicated, merely in part,by the few arrows 6. In particular, those electrons in the primarystream 1 which are scattered backward from the upper surface of theobject 2 are reflected down towards said upper surface of the object 2by the reflector 4a. Even though most objects to be sterilized are ofrelatively low atomic number, five or ten percent of the energy of theincident beam 1 may be scattered backward from the upper surface of theobject 2. When these scattered electrons are redirected down onto theupper surface of the object 2, by the reflector 4a, the region near theupper surface which, owing to the normally occurring dose distributionin the object 2, would otherwise receive a lower dose than that receivedby the rest of the object 2, now receive supplemental irradiation.Consequently, the dose distribution is rendered more uniform near thesaid upper surface of the object 2.

In addition, any electrons in the primary stream 1 which pass the object2 will be reflected back towards the object 2 by the reflector 4a. Thisis illustrated more clearly by the modified form of apparatus shown inFigs. 3 and 3A, by which the method herein disclosed may be practiced,also, and wherein the object 2 to be sterilized is supported upon amoving belt 40 by means of any suitable support, such as indicated at 3.Alternatively the object 2 may rest directly upon the moving belt 40.The upper surface 9 of the belt 40 is a. material of high atomic number,and serves to refiect back towards the object 2, as indicated by onearrow 5, those electrons in the primary stream 1 which pass the object 2as well as some of the electrons which are scattered out of the object'2. In addition, two stationary reflectors 4b are positioned over theobject and alongside the incident beam 1, and serve to reflect backtowards the object 2 those electrons which are scattered out of theupper surface thereof. Such stationary reflectors 4a are desirably ofthe length or" the object 2, as indicated in Fig. 3A.

Having thus described several embodiments of the apparatus of myinvention, and by each of which the method of my invention may bepracticed, it is to be understood that although specific terms areemployed, they are used in a generic and descriptive sense and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

I claim:

1. That method of increasing the treatment range of an available beam ofhigh energy electrons in electron sterilization, which comprisesbombarding an object to be sterilized from one aspect with a beam ofhigh energy electrons, permitting some of the electrons of such beam totravel beyond said object, and reflecting back by elastic nuclearscattering from a material of high atomic number onto such object asubstantial amount of those electrons that traveled in the bombardingaction beyond the object being bombarded.

2. That method of increasing the treatment range of an available streamof electrons in electron sterilization, which method comprisesbombarding an object to be sterilized from one aspect with a beam ofhigh energy electrons, directing said beam towards said object in such amanner that some of said electrons travel beyond said object, andreflecting said electrons which travel beyond said object, byelectrostatic interaction with the nuclei of atoms of a material of highatomic number, whereby said object is simultaneously bombarded byelectrons from opposing aspects.

3. That method of sterilizing an object of irregular shape byirradiation with high energy electrons, which method comprisesbombarding the object from one aspect with a beam of high energyelectrons, directing said beam towards said object in such a manner thatsome of said electrons travel beyond said object, and reflecting saidelectrons which travel beyond said object by electrostatic interactionwith the nuclei of atoms of a material of high atomic number, wherebysaid object is simultaneously bombarded from opposing aspects byelectrons traveling over a wide band of angles.

4. Apparatus for irradiating an isolated object with high energyelectrons, comprising means for creating a well-defined beam of highenergy electrons, means for directing from one aspect said beam onto anisolated object to be irradiated, and a reflector, of material of highatomic number, substantially enclosing said object, but having anaperture positioned to provide an unobstructed path for said beamdirectly to said object, whereby there are reflected back onto saidobject a proportion of those high energy electrons of said beam, that inthe irradiating action passed said object without touching it, or werereflected from said object by nuclear scattering.

5. That method of securing the more uniform administration of ionizingenergy with respect to an isolated object being irradiated by highenergy electrons, and of increasing the amount of ionizing energyabsorbed by such object, and of increasing the effective range ofpenetration of such high energy electrons; which method comprisescreating a beam of high energy electrons and directing the same onto theobject to be irradiated, and reflecting back toward such object byelastic nuclear scattering from a material of high atomic number thosehigh energy electrons which in the irradiating operation passedalongside said object without striking it.

6. That method of securing the more uniform administration of ionizingenergy with respect to an object being irradiated by high energyelectrons, and of increasing the amount of ionizing energy absorbed bysuch object, and of increasing the etlective range of penetration ofsuch high energy electrons; which method comprises creating a beam ofhigh energy electrons and directing the same onto the object to beirradiated, and reflecting back toward such object by elastic nuclearscattering from a material of high atomic number at least a substantialamount of those high energy electrons which have in the irradiatingoperation been scattered from its volume.

7. That method of securing the more uniform administration of ionizingenergy with respect to an isolated object being irradiated by highenergy electrons, and of increasing the amount of ionizing energyabsorbed by such object, and or" increasing the eliective range ofpenetration of such high energy electrons; which method comprisescreating a beam of high energy electrons and directing the same onto theobject to be irradiated, reflecting back toward such object by elasticnuclear scattering from a material of high atomic number those highenergy electrons which in the irradiating operation passed alongsidesaid object without striking it, and reflecting back toward such objectby elastic nuclear scattering from a material of high atomic number atleast a substantial amount of those high energy electrons which have inthe irradiating operation been scattered from its volume.

8. That method of securing the more uniform administration of ionizingenergy with respect to an isolated object being irradiated by highenergy electrons, and of increasing the amount of ionizing energyabsorbed by such object, and of increasing the eflective range ofpenetration of such high energy electrons; which method comprisescreating a beam of high energy electrons and directing the same onto theobject to be irradiated, causing to be redirected toward such objectthose high energy electrons which in the irradiating operation passedalongside said object without striking it, and causing to be redirectedtoward such object at least a substantial amount of those high energyelectrons which have in the irradiating operation been scattered fromits volume, by substantially enclosing said object by a reflector ofhigh atomic number material so as to redirect toward such object boththose high energy electrons which have, in the irradiating operation,passed alongside it without striking it, and also those electrons whichhave been, in the irradiating operation, scattered from its volume.

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